I read a thread regarding a 24'' rear wheel on a Twenty (I think it was little pixel that raise the idea) - I do think its possible to do it and thinks about doing it (Fixed Gear, new 24'' fork etc.) - but I really need to know what will be the affect on the riding ?

You'll have raised the bike by about 2" at the back and front if you put a 24 on there as well; I think the stability probably wouldn't be too affected as you'd get some extra gyroscopic effect with the larger wheels. Are you planning to remove the bridge on the chain and seat stays? The other consideration I suppose is clearance at the front so you dont get your toes in the way of the front wheel when you steer. Wheelbase should be long enough but it's worth checking. I look forward to seeing your 'Bigfoot' Twenty! Huw